Recently, a zinc oxide varistor has been used more and more in order to assure the safety and operating stability of electronic and electric appliances against, for example, various surge or pulse noise or induced lightning, and also to cope with reinforcement of various noise regulations. A conventional zinc oxide varistor will now be explained. The operating voltage of a zinc oxide varistor is the voltage at the time when 1 mA of electric current runs in the varistor. Depending on the types of circuits electronic and electric appliances have, the operating voltage ranges from several tens of V to several tens of thousand V. However, the operating voltage is basically determined by the number of particles present between electrodes, and it is known that the operating voltage per one particle is about 3 V. Except for high voltage products with high operating voltage, low voltage products have a fixed thickness, so that the operating voltage is dependent upon the diameter of particles, that is, the number of particles.
When ceramics are sintered, the density starts to increase in the initial stage without so much accompanying growth of particles. As soon as the increase stops, growth of particles, called a secondary recrystallization particle growth, will then take place. This process often creates abnormally large particles, which is called an abnormal particle growth, that causes distribution in operating voltage. Therefore, a product with operating voltage of 100 V and higher, which may have small particles, had around 33 particles and more in the 1 mm thickness. Statistically, this product comprised a proper mixture of large particles and small particles, and distribution in operating voltage did not become so large as to interfere with production. In the case of a low voltage product of 100 V and lower, it was necessary to allow particles to grow. Therefore, it was common to slow down the programming rate and to fire for many hours.
For the purpose of securing reliability, and also by estimating drifting during the firing, a zinc oxide varistor is provided with a large amount of oxide or carbonate to excess, so that additive elements are sufficiently present inside the particles and in the particle field. When the additives are fired, one of the compounds which can be present in the end is a solid solution of zinc oxide spinel, and these compounds which are present in the particle field affect the distribution in the diameter of particle. Furthermore, the increase of programming rate at the time of the debinder process (0.degree. to 650.degree. C.) can lead to various reliability deteriorations, for example, distribution in operating voltage and leakage current may increase because the debinder process is insufficiently performed and carbon remains. In addition, a rapid decomposition of binder can cause, for example, cracks in products, so that this method was not suitable as a manufacturing method in the industry.
However, in the conventional method mentioned above, giant particles of 100 .mu.m and larger which are trying to grow abnormally and smaller particles with insufficient particle growth are coexisting, and a large number of particles with about 20 .mu.m are also present. As a result, distribution in operating voltage were large, and it was difficult to attain the product specification of .+-.10% in operating voltage. Also, its low yield of around 50% was a big cost problem. Since additives which produce the solid solution of zinc spinel are present in the particle field in the form of microparticles, they prevent particles from growing, thereby causing distribution. Therefore, it is considered appropriate to control the amount of additives. However, it is necessary to add the additives to excess in order to secure reliability and to attain stable quality.